Vertebral fusion is a common spinal surgical procedure, performed in order to overcome problems related to impaired mutual interaction between neighboring vertebrae. A pair of adjacent human vertebrae is shown in FIG. 1. Each of the vertebrae 10 and 20 has, respectively, a vertebra body 11 (21), two pedicles 12 (22), two transverse processes 13 (23), two laminae 14 (24), a spinal process 15 (25), two superior facets 16 (26), and two inferior facets 17 (27). The body of the pedicle 12 is attached by its base end to the vertebra body 11, while its top end branches into the transverse process 13, the superior facet 16 and the lamina 14. The inferior facet 17 is attached to the lamina 14. The two laminae 14 are connected to either side of the spinal process 15 and, together with the pedicles 12 and the vertebra body 11, form a channel 18 sheltering the spinal cord. The superior facets 26 of the lower vertebra 20 and the inferior facets 17 of the upper vertebra 10 form two facet joints 19. A spinal disc 29 connects elastically each two adjacent vertebrae. (The same numerals will be used throughout to designate same parts of the vertebra.)
There are a number of indications requiring spinal fusion operations including:
(i) traumatic fracture of the vertebral body;
(ii) degenerative disc or vertebral disease, such as disc herniation, instability of the facet joint, compressive radiculopathy;
(iii) following the failure of previous spinal surgery, or the removal of a disc;
(iv) chronic vertebral or disc infection;
(v) vertebral instability, such as in spondylolisis or spondylolystetis; and
(vi) following the removal of spinal tumors.
The technique involves the disablement of the relative motion between adjacent vertebrae to prevent compression during body movements and for stabilizing the spinal column. Spinal fusion is a very common procedure, with over 400,000 such procedures performed annually in the USA alone.
Three methods are currently generally used for performing this procedure;
(a) Fixation of the rear of the vertebrae by means of hooks, two of which are generally used to hook onto the laminae on either side of the vertebrae, and connection between them by means of a metallic rod, as exemplified by U.S. Pat. No. 5,267,999. The method is illustrated schematically in FIG. 2 where a clamp 30 is fixed by hooks 32 and 34 to the lamina 14 of the vertebra 10.
(b) Pedicle screw insertion into the body of the vertebra, for example as described in U.S. Pat. No. 4,913,134. This method involves the insertion of two screws per vertebra, on the left and right spinal pedicles. A rigid rod or an elongated fusion plate is secured under the screw head on either side of the vertebra, thus preventing motion between adjacent vertebra.
The method is illustrated schematically in FIGS. 3A and 3B showing, respectively, cross-sectional and lateral sectional views of vertebral fusion using the pedicle screw procedure. Two pedicle screws 40 with special heads 42 are inserted through the pedicle 12 into the vertebral body 11. Fixing rods 44 are attached under the screw heads 42 to fuse together the vertebrae 10 and 20.
(c) Fixation of the front part of the vertebrae, either by means of screws inserted into the body of the vertebra, or by use of hollow threaded cages inserted into pre-drilled tracts between the vertebrae, and which are filled with bone graft material, which ultimately fuses with the vertebral bodies to consolidate the fusion.
The method is illustrated schematically in FIG. 4 which shows a frontal view of vertebral fusion using the threaded fusion cage procedure. Fusion cages 46 with bone graft material therein 48 are shown inserted between two vertebral bodies 11 and 21, from the front of the spine.
Each of these prior art methods has its own specific disadvantages:
(a) Hook fixation—because of the comparative weakness of the holding power of the hooks, a larger number of vertebrae need to be fused to achieve acceptable results, than when using the other methods. This may lead to unnecessarily limited movement of the spine in the operated region, with consequent problems of mobility. Furthermore, what should be a comparatively simple operation and localized operation becomes much more extensive, with concomitant increase in blood loss, time under anesthesia, and the need for an external brace for some time after the operation. Failure of the hooks to hold well may cause them to move during the healing period.
b) Pedicle screwing—the insertion of the pedicle screw is generally a difficult, error prone and time consuming procedure. The surgeon must determine the entry point and the trajectory of the screw holes from X-ray fluoroscopic images taken from several angles, and this requires expertise and experience, and a well-trained operating room team. Furthermore, the method results in a high level of X-ray exposure to the surgeon and staff. The following problems can arise during this procedure:
(i) Breakage of the pedicle due to a misdirected drill, or poor drilling technique.
(ii) Penetration of the spinal cord, with resultant nerve damage.
(iii) Damage to neighboring nerve roots, causing pain and nerve damage.
(iv) Penetration of the abdominal cavity, with the resultant danger of damage to major blood vessels and hemorrhaging.
(v) Part of the upper tip of the facet joint often needs to be removed to provide room for the pedicle screw head.
A number of clinical studies have reported 10 to 40% misplaced screws, which is defined as a screw more than 2 mm away from the intended ideal position. It has been reported by R. W. Gaines in the article “The Use of Pedicle Screw Internal Fixation for Spinal Disorders”, Journal of Joint and Bone Surgery, Vol. 82-A, No. 10, 2000, that about 3% of misplaced screws are more than 5 mm away from their planned position, almost inevitably causing nerve damage. Insertion of pedicle screws in the thoracic and cervical vertebrae is even riskier because of the compact and delicate structure of the spine in these regions. For this reason, few experienced surgeons perform spinal fusion by this means at these levels, and in the USA, the procedure is not even FDA authorized for these regions.
(c) Fusion cage—in order to drill the necessary tracts for inserting the cages, or the holes for inserting the screws, this procedure requires one of two major surgical procedures. Either access is needed to the front of the spine through the abdomen or thoracic cavity, with its concomitant possibilities of complications, or access to the vertebrae from the rear, in which case a wide laminectomy procedure must be performed, and displacement of the entire dura matter in the region of the cage insertion
Furthermore, in all of the methods, there are the general difficulties relating to surgery on the vertebral column, include micro-movement of the vertebral column during the operation, the inherently small target objects of the procedure such as the pedicles, and delicate nerve tissue close to the operation region.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.
Fixation of fractured tubular bone fragments is also a common orthopedic surgical procedure. It is performed on long bones of the limbs in order to ensure correct position of the fragments during their natural fusion and better mobility of the patient during healing. An example of internal fixation known in the prior art is shown schematically in FIG. 5. Such fixation of a broken bone 86 usually uses at least three screws 92, at each side of the fracture 94 to fix a supporting plate 96. The operation includes exposure of a rather long area of the bone.
There are also methods for external fixation of fractured bones, all of them involving application of long screws partially remaining above the skin surface to be fixed to the supporting plates. Such screws are likely to cause secondary infections of the operated limb.